from PyQt5.QtWidgets import QApplication, QLabel,  QWidget, QPushButton, QVBoxLayout, QMessageBox

def on_button1_clicked():
    alert = QMessageBox()
    alert.setText('You clicked the button')
    alert.exec_()

if __name__ == '__main__':
    app = QApplication([])
    # label = QLabel('Hello World!')
    # label.show()

    app.setStyle('Fusion')
    window = QWidget()
    window.resize(400, 300)
    layout = QVBoxLayout()
    layout.addWidget(QPushButton('Top'))
    layout.addWidget(QPushButton('Bottom'))

    # clicked
    button1 = QPushButton('Clicked')
    button1.clicked.connect(on_button1_clicked)
    layout.addWidget(button1)

    window.setLayout(layout)
    window.show()

    app.exec_()